Indium oxide/n-silicon heterojunction solar cells

ABSTRACT

A high photo-conversion efficiency indium oxide/n-silicon heterojunction solar cell is spray deposited from a solution containing indium trichloride. The solar cell exhibits an Air Mass One solar conversion efficiency in excess of about 10%.

BACKGROUND OF THE INVENTION

The present invention is the result of research performed for theDepartment of Energy under U.S. Government Contract XJ-0-9077-1.

This application is a continuation-in-part of application Ser. No.222,367, filed Jan. 5, 1981, now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to heterojunction solar cellsand more particularly to an efficient indium oxide/n-silicon solar cellexhibiting sunlight conversion efficiencies in excess of about 10%. Arelatively simple spray hydrolysis is utilized to form the indium oxidesemiconductor, deposited onto an oxidized surface of a siliconsubstrate.

Low cost alternatives to conventional diffused junction silicon solarcells are continually sought to provide viable alternatives to fossilfuel generation of electricity. Low cost processing and fabricationtechniques must accompany a threshold solar cell efficiency to befiscally attractive, for attendant solar cell costs such as land areacoverage etc. indicate that a solar cell having an efficiency less thanabout 10%, irrespective of its intrinsic cost, would not be commerciallyviable. The present invention is directed to a solar cell satisfyingthese criteria having both high efficiency and relatively low processingand intrinsic material costs.

The art has well recognized the semiconductor properties of degenerateoxides such as tin oxide, indium tin oxide, and cadmium stanate. Tinoxide has been successfully employed in the fabrication of highefficiency low cost solar cells as illustrated in the applicants' priorinvention, U.S. Pat. No. 4,193,821. Indium oxide has been used to formheterojunction solar cells with p-type silicon. Such indium oxide solarcells have previously met with limited success as compared to their tinoxide counterpart principally because they have solar conversionefficiencies less than about 5% and exhibit long-term stabilityproblems. The prior art has taught a construction of indium oxideheterojunction with p-type silicon whereas the present invention isdirected to the formation of the heterojunction between indium oxide andn-type silicon having a thin surface layer of silicon oxide. Theprevious state of the art is best illustrated in a research report(NSF/RANN/SE/AER74-17631/FR/75/3) to the National Science Foundation inwhich R. L. Anderson et al. noted that indium oxide/p-type silicon (In₂O₃ /p-Si) solar cells exhibit conversion efficiencies of 3-4% with amaximum conversion efficiency found to be about 4.9%. In addition, theyhave reported that the In₂ O₃ /n-Si cells that were fabricated by usingn-type silicon had poor solar cell characteristics and on the basis ofan analysis of the relative electron affinities, they concluded that In₂O₃ /n-Si cannot be a practical device. In a technical publication in theJapanese Journal of Applied Physics (Vol. 14, No. 6 (1975), p. 915), H.Matsunami et al. have reported that good rectification and photovoltaiceffects can only be obtained with In₂ O₃ /p-Si cells but not with In₂ O₃/n-Si cells. The indium oxide/n-silicon heterojunction solar cells ofthe present invention, not only far exceed the efficiency of suchexisting In₂ O₃ /p-Si heterojunction cells, but furthermore, providerelatively simple construction techniques promising a substantialreduction in the overall cost of the solar cells.

SUMMARY OF THE INVENTION

The invention is directed to a high efficiency indium oxide/n-siliconheterojunction solar cell and method for making same. A solutioncontaining indium trichloride (InCl₃) is sprayed in air onto the surfaceof a preheated silicon substrate, where the indium chloride ishydrolyzed to form an indium oxide (In₂ O₃) film. The depositiontechnique employs relatively simple, less energy intensive technology ascompared to conventional junction diffusion techniques. The resultingheterojunction solar cell exhibits an Air Mass One solar conversionefficiency in excess of about 10%.

BRIEF DESCRIPTION OF THE DRAWING

The singular drawing is a greatly enlarged side-segmented view of aheterojunction solar cell made in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the FIGURE, the solar cell of the present inventioncomprises an n-type silicon substrate 10 having a relatively thinsilicon oxide layer 12. Silicon substrate 10 can be eithermonocrystalline or polycrystalline in form and having a resistivitygenerally below about 10 ohm-cm. In a preferred embodiment, siliconsubstrate 10 comprises a single crystal silicon wafer ranging inthickness of about 5 mils to about 15 mils and having a resistivityranging from about 0.1 ohm-cm to about 1.0 ohm-cm. To enhance theopen-circuit voltage, thus the efficiency, of the cell, a thin SiO₂layer 12 having a thickness in the range of 10 to 30 Angstroms isincorporated. Such a silicon oxide layer can be grown, for example, byheating the silicon in air at an elevated temperature. However, in mostinstances, a "naturally-grown" SiO₂ layer is of sufficient thickness andis preferred. The indium oxide (In₂ O₃) layer 14, contiguous with theoxidized silicon substrate, forms a heterojunction to the underlyingn-type silicon. In addition, layer 14 is characterized as being of aspecific thickness and having an index of refraction to act as ananti-reflection coating thus suppressing the reflectivity of the siliconsurface. As understood by those skilled in the art, an anti-reflectioncoating should consist of a film of low refractive index and the opticalthickness of the film should be an integer multiple of λ/4, where λ isthe wavelength of the incident light. To be highly effective, however,the refractive index of the anti-reflection film should be approximatelythe square root of the refractive index of the underlying substrate. Asemployed in the present invention, silicon substrate 10 and indium oxidelayer 14 have indices of refraction of about 4 and 2, respectively. Tosubstantially reduce the reflection of solar radiation from the surfaceof the silicon substrate, the thickness of the indium oxide layer 14 ischosen to result in a blue interference color. Depending on the order ofinterference, various specific thicknesses of indium oxide film willresult in a blue interference color (see for example, J. L. Vossen,"Transparent Conducting Films", in Physics of Thin Films, Vol. 9, Editedby E. Hass, M. H. Francombe, and R. W. Hoffman, Academic Press, N.Y.,(1977), p. 18.)

Indium Oxide Layer 14 is further characterized as being semitransparent,permitting a sufficient amount of light energy incident thereupon topass therethrough and into the silicon. As presently understood, in theformation of the heterojunction between the indium oxide and theoxidized silicon substrate the depletion region or barrier region iscontained within the silicon substrate 10. Furthermore, charge carriersgenerated within the silicon substrate, outside the depletion region,will migrate to the field region to effectively contribute to thecurrent generation of the solar cell. It is accordingly required that asubstantial amount of light pass through the indium oxide layer 14 andbe absorbed in the silicon substrate. Thereby, according to one aspectof the present invention, indium oxide layer 14 has a thickness lessthan about 5000 Angstroms. In addition, because suppression of thesurface reflection becomes more selective with wavelength as theinterference order increases, in a preferred embodiment the thickness ofthe indium oxide film 14 is chosen to correspond to a first orderinterference, that is, between approximately 600 and 1400 Angstroms.

Ohmic electrodes 16 and 18 comprise highly conductive materials,generally metals. Ohmic electrode 18, which is illustrated in part bythe FIGURE's side view, comprises a grid configuration permitting asubstantial amount of incident light to pass between the gridconfiguration. As recognized by those of the art, the grid configurationis utilized where the underlying semiconductor material is ofinsufficient electrical conductivity to support the current generated bythe solar cell. The geometric configuration of grid 18 may comprise anyof the number of alternatives which attempt to optimize surface coverageand minimize series resistance of the solar cell. Parameters leading tothe optimization of the grid configuration include the currentgeneration characteristics of the heterojunction, resistivity ofelectrode material utilized to configure grid 18, and theelectro-resistivity of indium oxide layer 14. In a preferred embodiment,an adherence layer 17 bonds the grid material 18 to the oxide. A thinfilm of titanium has been demonstrated to serve as an effective bondingor adherence layer for conductive grids such as silver for example.

As discussed heretofore, the present solar cell technology incurssubstantial expenditures in the processing of the solar cell. Accordingto one aspect of the present invention, a relatively simple means forproducing the indium oxide/n-silicon solar cell promises substantialcost reduction in the fabrication of efficient, inexpensive solar cells.The process comprises heating the silicon substrate in air at atemperature ranging from about 350° C. to about 450° C. Upon reachingthe required temperature, the heated substrate is contacted with anatomized liquid solution containing indium trichloride (InCl₃) whichwill hydrolyze upon contact with the heated substrate and, in thepresence of air, to form the indium oxide layer of the presentinvention. Generally described, the atomizing liquid solution comprisesanhydrous indium trichloride, water, and an organic ester, and/or analcohol having boiling points of about less than 250° C. Examples ofalcohols and organic esters suitable for use in the present inventioninclude: methyl alcohol, ethyl alcohol, ethyl acetate, butyl acetate,amyl acetate, and propyl acetate. Anhydrous indium trichloride is notreadily soluble in alcohol or organic ester and requires firstdissolution in water, followed by the addition of the remainder of thesolvent. The amount of water employed is generally sufficient todissolve the indium trichloride. The remainder of the solvent maycomprise either an ester or an alcohol. However, it is particularlypreferred that solutions having an about one to one volume ratio of anester and alcohol be employed. A solution concentration of indiumtrichloride between 0.05 molar to 5 molar is preferred.

In atomizing the liquid solution, any well-known atomizing device may beused. For example, electrostatic, pneumatic, or vibrational atomizingmay be used to provide a spray or mist of the liquid solution of indiumtrichloride. It is particularly preferred in the practice of the presentinvention to use a pneumatic spraying device, in which the gas used toatomize the liquid solution is air. It should be appreciated however,that other carrier gases, including oxygen, may be employed.

As is well-known, spray guns typically subdivide liquid solution intodroplets having diameters generally in the range of 100 to 1,000microns, whereas misting devices generate or subdivide liquid solutionsinto droplets having diameters in the range of 10 to 100 microns. Whilethe size of the droplets produced during the atmoziation of the indiumtrichloride solution has not been demonstrated to be critical, it isdesirable that the droplets be generally less than about 1,000 micronsin diameter. It is also generally desirable that the spray or mist beuniform across at least the area to be coated. Conventional liquid spraydevices produce such an atomized stream of solution.

The contacting of the heated silicon substrate with the atomizedsolution is continued for a time sufficient to form the requisitethickness of an indium oxide layer. Although numerous thicknessmeasuring techniques are known in the art, the deposition techniquetaught in the present invention is of particular advantage inasmuch as avisual observance of the interference color of the film duringdeposition provides a convenient and relatively accurate means formonitoring the thickness of the depositing indium oxide layer.Accordingly, the deposition continues until an interference color ofdeep blue is observed. A sufficient and preferred thickness of indiumoxide ranges from about 600 A to about 1400 A, although thicker layerscorresponding to higher order interference are also operable in thepresent invention.

The silicon substrate is continually heated throughout the deposition ofthe indium oxide layer. It is to be recognized that the temperaturerange as recited herein represents a temperature monitored at thesubstrate surface prior to spraying by means of a surface probe such asa digital temperature indicator, Model 64-06-02 sold by Watlow Corp.,Winona, Minn. Reference is made to the substrate surface temperatureherein and there can be significant difference in the temperature of thesubstrate surface and the temperature of the furnace employed forheating the substrate. For example, it has been found that the surfacetemperature of the silicon substrate heated by a graphite vacuum chuckonly reached 405° C. while the temperature inside the heating chuck wasmonitored at 430° C. Although it is not necessary to monitor thesubstrate surface temperature once the relationship between the surfacetemperature and the furnace temperature is established, nonethelessreference to temperature of substrate surface is made herein to assistone skilled in the art in the practice of the present invention.

The time required for the deposition of the indium oxide layer isgenerally less than 2 minutes and is typically of the order of 40seconds. As should be readily appreciated, the time required fordeposition of an adequate layer of indium oxide will depend upon theconcentration of the solution, the flow rate of the solution and thetemperature of the substrate. According to one aspect of the presentinvention, upon completion of depositing the indium oxide layer thesilicon substrate is removed from the heat source to rapidly lower thetemperature of the device.

After deposition of the indium oxide, the coated silicon substrate wasprovided with appropriate electrodes. For example, back contactmetallization is accomplished by standard techniques such as vacuumevaporation of 1000 Angstroms of titanium followed by 5000 Angstroms ormore of silver to provide a good ohmic contact to silicon. Front contactmetalization described heretofore, is also accomplished by standardtechniques, except that the titanium and silver are evaporated through asuitable mask with appropriate grid patterns.

The solar cells constructed in conformity with the present inventionexhibit a solar Air Mass One conversion efficiency greater than about10%.

To further assist one skilled in the art in the practice of the presentinvention, the following example details a specific embodiment of theinvention.

EXAMPLE 1

Following the general procedures outlined hereinabove, a series ofindium oxide/n-silicon solar cells were fabricated and the sunlightpower conversion efficiencies were measured.

Single crystal silicon wafers having the general characteristics listedbelow were used: N-type (phosphorous doped), 0.1-0.5 ohm cm, (100)orientation, 15 mils thick, front surface polished, and back surfaceetch damaged. The wafers were typically used as received with noadditional cleaning. Therefore, the silicon had a SiO₂ layer on thesurface grown "naturally" at room temperature, generally of the order ofbelow about 30 Angstroms in thickness.

A silicon wafer, with polished side facing upward, was placed on agraphite vacuum chuck. The vacuum capability of the chuck is utilized tofirmly secure the silicon wafer to the heating source during the spraydeposition discussed hereinafter. The chuck was heated by 3 Ogden (ModelNo. MW-63) cartridge type heaters embedded in the graphite, and thetemperature is controlled by means of a Love (Model 49) temperaturecontroller. The temperature of the graphite vacuum chuck was raised to450° C. as measured by a thermocouple embedded in the graphite. However,as noted previously there is a temperature differential between thesurface temperature of the silicon substrate and the chuck. Maintainingthe chuck at 450° C. was found by a surface temperature probe to providea silicon substrate surface temperature prior to spraying of about 425°C. The spray solution comprised 25 grams of anhydrous indium trichloridedissolved in 25 cc of water to which 125 cc of ethyl acetate and 125 ccof methanol was added. The spray solution was atomized by means of aBinks pneumatic spray nozzle (Model No. 50-175). The distance from thenozzle to the silicon surface was about 28.5 centimeters. Once thetemperature of the graphite vacuum chuck had stabilized at 450° C., theatomized InCl₃ solution was sprayed onto the heated silicon substrate.The liquid flow rate of the InCl₃ spray mixture was measured by a flowmeter to be about 10 cc/min.

Throughout each deposition the interference color of the indium oxidelayer was monitored to provide a reasonably accurate indication of thethickness of the depositing oxide layer. Upon reaching an indium oxidethickness having an interference color of deep blue, the deposition wasterminated and the device quickly removed from the heating source torapidly lower its temperature. The spray deposition time in thisparticular example was 70 seconds. The oxide thus formed is stronglyadherent and resistive to physical abrasion.

The indium oxide coated wafer was then transferred to a vacuumdeposition chamber where standard electron-beam heated evaporationtechniques were utilized to deposit an ohmic electrode comprising 1000Angstroms of titanium followed by 5000 A of silver to the back surfaceof the silicon substrate. The coated wafer was then removed from thevacuum chamber and the indium oxide film was placed in intimate contactwith a metal evaporation mask with multiple grid patterns. It was thenplaced back in the vacuum system. And after pumping down to a goodvacuum, 10,000 Angstroms of silver was evaporated through the mask ontothe indium oxide film. Subsequently after removing from the vacuumchamber, the wafer was cut into individual 1- and 4-cm² cells of thetype shown in the FIGURE.

The efficiency, short-circuit photocurrent density, open circuitphotovoltage, and fill factor of a typical cell measured in a simulatedAM1 solar spectrum are 11.1%, 28.6 mA/cm², 0.58 V, and 0.67,respectively.

What is claimed is:
 1. A high efficiency indium oxide/n-siliconheterojunction solar cell comprising:an n-type silicon substrate havingat least one major area surface coated with a thin layer of siliconoxide, SiO₂ ; a layer of indium oxide deposited onto said silicon oxideand forming a heterojunction therewith and concurrently forming ananti-reflection layer to said surface; ohmic contact means for makingelectrical connection to said silicon and said indium oxide layerswhereupon illumination said solar cell with an equivalent of Air MassOne solar energy, said solar cell exhibits a conversion efficiency inexcess of about 10%.
 2. The solar cell of claim 1 wherein said siliconsubstrate has a resistivity ranging from about 0.1 ohm cm to about 1 ohmcm.
 3. The solar cell of claim 2 wherein said silicon substratecomprises either single-crystal or polycrystalline silicon.
 4. The solarcell of claim 1 wherein said ohmic contact to the indium oxide comprisesa grid electrode covering a portion of the major surface area of saidoxide.
 5. The solar cell of claim 4 wherein said grid electrodecomprises titanium and silver.
 6. The solar cell of claim 1 wherein saidohmic contact means connected to the silicon comprises titanium andsilver.
 7. The solar cell of claim 1 wherein the indium oxide layer isof a thickness characterized by an interference color of deep blue. 8.The solar cell of claim 1 wherein said thin layer of silicon oxide, SiO₂is less than about 30 A.